1. Field of the Invention
This invention relates to a method for treatment of a spent caustic stream and in particular to treatment of the spent caustic stream for removal of organic contaminants to condition the caustic stream for more efficient and complete oxidation that better conditions the caustic stream for disposal.
2. Description of the Related Art
In the petroleum and petrochemical industries it is common to scrub gas mixtures that contain acid gas components, such as carbon dioxide (CO.sub.2) and hydrogen sulfide (H.sub.2 S), to remove these components from such gas mixture before it is used for further processing purposes or otherwise disposed of as by venting to the atmosphere. An aqueous sodium hydroxide solution--i.e., a caustic solution--is commonly used for scrubbing of such gas mixtures. By reaction with the caustic solution, i.e. NaOH, acid gas components such as hydrogen sulfide and carbon dioxide are converted into sodium sulfide (Na.sub.2 S), sodium hydrosulfide (NaHS), sodium carbonate (Na.sub.2 CO.sub.3) and sodium bicarbonate (NaHCO.sub.3) which carry into the sodium hydroxide (NaOH) solution. Wherein the gas mixture to be scrubbed also contains hydrocarbon components (particularly C.sub.4, C.sub.5 and higher molecular weight hydrocarbon) a portion of these hydrocarbon components also pass as such into the aqueous sodium hydroxide stream, each to the limit of its mutual solubility in solution.
One type of petrochemical operation wherein an aqueous sodium hydroxide solution is almost invariably used for gas scrubbing is in an ethylene production unit. In an ethylene production unit a saturated aliphatic hydrocarbon feed, such as ethane, propane or higher molecular weight hydrocarbon mixtures such as naphtha, atmospheric and/or vacuum gas oil, and the like, is heated at high temperatures in the presence of steam to crack the saturated hydrocarbon molecules down to lower molecular weight unsaturated hydrocarbons such as ethylene predominately, followed by propylene, and then various quantities of C.sub.4, C.sub.5 and C.sub.6 mono- and diolefinic hydrocarbons, with a lesser quantity of C.sub.7 and higher molecular weight saturate and unsaturate aliphatic, alicyclic and aromatic hydrocarbon. During steam cracking, any sulfur containing compounds present in the hydrocarbon feed stream are converted into hydrogen sulfide and/or organically bound sulfur compounds and also a content of carbon dioxide is generated in the cracked gas mixture by the water gas shift reaction. The resultant gas mixture from steam cracking is then quenched to a lower temperature of from about 35 to 40.degree. C., whereupon the major portion of its water and C.sub.7 hydrocarbon content is condensed and separated from said gas mixture. After quenching, the remaining constituents of the gas mixture are conditioned by various steps of gas compression and refrigerative cooling to prepare it for cryogenic distillation whereby its ethylene, propylene and butenes contents will ultimately be recovered in essentially pure form for ultimate use as monomers in the production of various polymers, such as polyethylene, ethylene copolymers, polypropylene and the like.
One step required to properly condition the cracked gas prior to its cryogenic distillation is to scrub the cracked gas essentially free of any acid gas components, such as hydrogen sulfide and carbon dioxide. This is accomplished at some interstage location of a multi-stage gas compression system and, on occasion post-compression, wherein the cracked gas stream is at a pressure from about 10 to about 30 atmospheres (atm) by contacting the compressed cracked gas stream with an aqueous sodium hydroxide solution by countercurrent contact in a gas-liquid contact vessel often referred to as an "absorber" or "scrubber."
The aqueous sodium hydroxide solution after such gas scrubbing contact is referred to as a "spent caustic solution" and contains, in addition to sodium hydroxide, the sodium sulfide, sodium hydrosulfide, sodium carbonate and sodium bicarbonate that results from the removal of acid gas compounds from the so scrubbed cracked gas stream and also a significant content of dissolved aliphatic, mono- and di-olefinic, as well as cyclic hydrocarbon and various carbonyls, styrenics and other organic contaminants. In this condition the spent caustic stream presents various problems with respect to its environmental disposal. For example, polymers tend to form in the spent caustic solution as long as the solution contains dissolved polymer precursors at an elevated temperature. Aldol condensation of dissolved oxygenated hydrocarbons (carbonyls, such as aldehydes and ketones) produces polymeric products that are commonly referred to as a "red oil," which is and remains partially soluble in a spent caustic solution that issues from the caustic scrubbing tower. Certain highly unsaturated hydrocarbons in the cracked gas, such as acetylenes and dienes (diolefinic hydrocarbons), that pass into the caustic solution in the scrubber may undergo addition type polymerization to various degrees., even to the point of a molecular weight which renders certain polymer species insoluble in the spent caustic solution such that they precipitate out of solution together with the aldol condensation polymers and may be removed from the spent caustic stream in a deoiling drum. In any event, the spent caustic solution removed from the scrubber, even following a deoiling drum treatment, contains in dissolved form a content of such condensation and addition types of prepolymer and polymer species which may later precipitate from the caustic solution as foulants of equipment surfaces that are later exposed to the spent caustic solution. From a disposal standpoint the sodium sulfide, sodium hydrosulfide contaminants as well as the dissolved hydrocarbon and other organic contaminants impart to the spent caustic stream too high of a chemical oxygen demand (COD) and/or biological oxygen demand (BOD) to allow for its environmentally acceptable disposal.
Accordingly, to reduce its COD and/or BOD, spent caustic streams are commonly subjected to an oxidation process to oxidize its organic contaminants and to oxidize its sulfide salts content to at least thiosulfates, and preferably to their highest oxidation state compounds. Such oxidation processes include wet air oxidation ("WAO") processes wherein an oxygen containing gas, such as air, is contacted with spent caustic at an elevated temperature in a contacting column. In this context, the dissolved hydrocarbon prepolymer and polymer contaminants in the spent caustic cause major problems, particularly with respect to spent caustic streams issuing from the operation of an ethylene production unit. Specifically, heat exchanger surfaces and other interior working surfaces, such as in transfer lines and valves, in a WAO process that are exposed to direct contact with the spent caustic undergoing WAO treatment tend to become clogged and fouled with polymeric materials over time, which necessitates periodic shutdown and cleanup of the WAO unit. Therefore, it is desirable to first free the spent caustic from dissolved polymers and polymer precursors if polymer formation and fouling of a WAO unit is to be avoided.
Proposals have been set forth in the art for methods of pretreating the spent caustic, prior to its oxidizing treatment, that are intended to reduce this fouling problem. For example U.S. Pat. No. 5,268,104 proposes to first contact an ambient temperature spent caustic with gasoline in a mixing drum and then separate the spent caustic from the gasoline in a deoiling drum after which the spent caustic, from which 70-100% dispersed oil has purportedly been removed, is oxidized with an air/ozone mixture. Even so, in practice a spent caustic pretreated by this mixing drum-deoiling drum technique has still been found to present a fouling problem to the equipment surfaces of post-treatment units. U.S. Pat. No. 5,244,576 by DeRoeck et al. proposes a somewhat more elaborate method for contacting a spent caustic stream with a recirculating stream of pyrolysis gasoline in order to remove prepolymer and polymeric hydrocarbons from the spent caustic prior to its treatment in a WAO unit. DeRoeck Patent '576 proposes to reduce polymeric fouling of the operating surfaces of a wet air oxidation unit by first intimately contacting spent caustic solution for a prolonged contact time with a recirculating volume of a pyrolysis gasoline as solvent to remove polymerizable hydrocarbon, particularly partial polymers, from the spent caustic. As described, the solvent is recirculated to the contacting vessel containing spent caustic at a rate of from 0.5 up to 10 times the volume rate of spent caustic under conditions that provide for a contact residence time of 10 to 20 minutes. Further, as the solvent is recirculated there is continuously both removed a take-off cut of solvent for solvent recovery and added a makeup quantity of fresh solvent, both in similar volumes, such that the volume ratio of fresh make-up solvent to spent caustic is about 1 to 100. Intimate contact of solvent with spent caustic is accomplished by the agitation created by the forced recycle of solvent using jet mixers or spray nozzles or by a mechanical stir. The vessel for contact may be subdivided, or a series of contact vessels may be utilized, to provide for multiple mixing stages or even a series of static mixers.
The procedure described by DeRoeck '576 is believed to be the state of the art pretreatment for spent caustic, achieving a significantly better removal of prepolymer and polymer organics from a spent caustic than simple mixing drum-deoiling drum treatment as described in U.S. Pat. No. 5,268,104. Therefore, the DeRoeck '576 procedures extend the operating time before polymer fouling requires shutdown and cleanup of the WAO unit. However, it has been found in practice that polymer fouling still presents a substantial problem with a spent caustic pretreated by the DeRoeck '576 procedure.
There is needed a still better, more efficient method for the treatment of a spent caustic stream to eliminate from it those contaminants which are objectionable from a standpoint of either its proper disposal or subsequent treatment to further condition the spent caustic for its safe disposal. Further, there is a need for a procedure for the oxidation of a spent caustic to higher conversion of sulfides to sulfates than can be economically achieved by wet air oxidation alone, in order to still further reduce the COD and BOD of the spent caustic before its disposal.